The present invention relates to modulators, and more particularly to a circuit for generating an amplitude modulated carrier signal by selectively combining plural carrier signals.
In AM radio bradcasting, a high power vacuum tube is conventionally used in the final radio frequency (RF) amplifier stage of the transmitter. The vacuum tube is usually biased for class C or class D operation in order to achieve high efficiency of operation. The RF amplifier does then operate quite efficiently (on the order of 75% efficiency), but also inherently requires a large amount of modulation power. For example, a five kilowatt RF amplifier of this type requires nearly 2.5 kilowatts of audio modulating signal power. High power audio amplifier stages are used to amplify the audio modulating signal to the required levels.
In the art these high power audio amplifiers are referred to as "modulators". This is somewhat different than the classical meaning of the term "modulator", since the actual RF carrier modulation occurs in the final RF amplifier stage rather than in the audio amplifier stage. To avoid confusion the term "modulator" is instead used herein in its classical sense to mean a device for varying a characteristic of a carrier signal.
Past attempts to improve the overall efficiency of operation of the RF modulating systems have been directed to improving the efficiency of the high power audio amplifier stages. It would be preferable, however, if an RF modulation system could be devised which did not require such high levels of audio modulating power in the first place.
The situation is somewhat analogous in the TV broadcasting art. Video signals are broadcast as AM signals. Class C and D amplifiers are not used in the final RF amplifier stages of TV transmitters, however, because they are nonlinear. Class AB amplifiers, which are linear, are therefore usually used instead. Although class AB modulators do not require that the modulating signal be provided at a substantial power level, they are relatively inefficient in operation, exhibiting efficiencies on the order of 20%. Efficiency of operation is important, however, since TV transmitters may have transmitter power output ratings as high as 1 megawatt. Needless to say, the cost of the electricity required to power these transmitters is significant. It would be desirable if a linear amplitude modulator of high efficiency could be developed.
An additional difficulty with prior RF modulator designs relates to their incompatibility with solid state amplifier elements. As mentioned above, the final RF amplifier stages of high power amplitude modulators have historically employed large vacuum tubes. It would be desirable if these vacuum tubes could be replaced by solid state amplifier elements. Solid state amplifier elements, such as BJT's, FET's, SCR's, etc. are preferred over vacuum tube devices in view of their size, relative efficiency, and reliability. Unfortunately the semiconductor elements currently available cannot readily be incorporated into past RF modulator designs.